Jupiter's four largest moons are shown to scale next to the giant planet's Great Red Spot, a storm system that is wider than Earth, in this composite image. From left, the moons are Io, Europa, Ganymede, and Callisto. Jupiter is the center of a mini-solar system, with its own entourage of moons, a system of rings, and an internal heat source. [NASA/JPL]

A solar-system-in-miniature tags along with the Moon tonight — a system anchored by Jupiter. The planet looks like a brilliant cream-colored star. It’s a little below the Moon as night falls, and stays with the Moon as they arc high across the south during the evening, then set in the wee hours of the morning.

After the Sun, Jupiter is the largest object in the solar system — about a tenth of the diameter of the Sun. And it’s encircled by more than 60 moons. The largest of them is bigger than the planet Mercury, and several of them are interesting worlds in their own right. One is covered by volcanoes, for example, while another has an ocean of liquid water beneath its icy crust.

Jupiter dominates this collection of worlds in the same way the Sun dominates Jupiter and the other planets — through its gravitational pull. It holds that entourage of moons in orbit like a miniature solar system.

Jupiter’s gravity is so strong, in fact, that it heats the planet’s interior to thousands of degrees, so Jupiter bathes its moons with infrared energy. But its gravity isn’t strong enough to squeeze the interior tightly enough to trigger nuclear fusion — the process that powers the stars. That’s because, despite its great heft, Jupiter isn’t massive enough to become a star. It would have to be about 80 times heavier for gravity to heat its interior enough to ignite the fires of nuclear fusion — giving our entire solar system the glow of a second star.

Script by Damond Benningfield, Copyright 2011

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